It is known that in a device of the laser-assisted tomographic atom probe type, a DC voltage is applied to the material specimen to be analyzed. This specimen takes the form of a machined tip, brought to a given electrical potential, the end of which is illuminated by focussed laser pulses, the energy supplied by the pulses causing the evaporation of the atoms constituting the material. According to a known principle, field evaporation is thus brought about by the electric field generated by the interaction of the laser pulse with the tip. In practice, the intensity of the laser pulse must be determined so as to provide for as brief a time as possible, the energy just necessary to evaporate an atomic layer fraction atom by atom.
It is known that too great an energy and/or too long a pulse duration have the known consequences on the one hand of altering the mass spectrum obtained due to altering the resolution (presence of spectral trails) and on the other hand, notably in the case where the intensity of the pulse is too great, to lead to the destruction of the specimen by both thermal and electrostatic effect.
Accordingly the person skilled in the art who implements a laser atom probe is faced with the tricky problem consisting in determining a minimum intensity level of the pulses which is both sufficient to guarantee evaporation of the elements constituting the material of the tip (and therefore the analysis of the material) and insufficient to give rise to an unacceptable degradation of the mass spectrum and to give rise to destruction of the specimen.
However, this optimal value of intensity is difficult to determine. Indeed, it is known that this optimal value is dependent on several parameters. It depends at one and the same time on the wavelength of the pulses emitted, the composition of the material analyzed and the exact geometry of the specimen tip (radius of curvature, cone angle, dimensions and regularity of shape).
Now, the exact composition of the analyzed material is on principle not known exactly, otherwise the analysis of the mass spectrum would not be necessary. Likewise, neither is the exact geometry of the material specimen rigorously controlled. Furthermore, the geometry of the specimen varies in the course of the analysis on account of the loss of material caused by evaporation.
Consequently, in the current state of the art, the obtaining of laser pulses having the optimal intensity requires empirical adjustment and the person skilled in the art is generally compelled, having regard to the laser source employed, to undertake the adjustment of the intensity of the laser pulses by trial and error. Furthermore, to perform the analysis of various specimens of materials, he is generally compelled to carry out a specific adjustment for each specimen analyzed, even in the case where the specimens analyzed are made from one and the same material. Moreover, because the geometry of the specimen varies in the course of the analysis, the person skilled in the art is generally also compelled to modify the intensity adjustment during analysis so as to preserve the quality of the mass spectra.
Hence, for the person skilled in the art, to carry out repetitive analyses making it possible for example to check that a method for making a composite material does indeed provide a material of constant composition, turns out to be a difficult and irksome operation which sometimes leads to the destruction of certain specimens and which gives rise to their replacement.